Lake Arrowhead Is Full

Special to the Mountain News

Editor’s Note: Ralph Wagner is retired from his long career as a professional hydrologist and he serves on the Arrowhead Lake Association Board of Directors.

Due to precipitation in February, Wagner said on March 3: “It will only take about 3½” of precipitation to fill the lake, so it will very likely occur this week of March 4-11.”

On March 5, Wagner called the Mountain News and confirmed:  “Sometime during Tuesday night, the lake is going to overflow. It’s not going to be anything spectacular. A third of an inch of rain will fill the lake up along with the runoff. One or two inches of rain and there will definitely be an overflow. Right now I don’t anticipate we’re going to have to open the gate, just let it overflow to the crest of the weir. We’re not going to rise that much and we want to save as much water as possible.”

To explain how the lake reached this point, Wagner prepared the following report for ALA:

Total precipitation in February 2019 was 20.46” as measured in my gauges on the North Shore of Lake Arrowhead. Gauges at other locations most probably will have measured more or less — on the South Shore at least 10% more.

Over 126 years of record, the average precipitation for the month of February is 7.73”, so February precipitation was 264.3% of the average for the month. This is a good sign that we are in a mild El Niño condition, since the 12.09” of precipitation in January 2019 was 158% of the average of 7.60” for the month of January.

Since record keeping started in the 1892-93 Water Year, there have been about 22 El Niño events at Lake Arrowhead. A Water Year is any 12-month period running from Oct. 1 of any given year until Sept. 30 of the next year. On average, El Niño events happen about every five to seven years, but sometimes they can be back-to-back, two to three years apart, or as much as 10 to 13 years apart.

Since the 1997-98 El Niño, they have been six to seven years apart. The last one was in Water Year 2010-11, so we are statistically due for one now, but it will still depend on what happens in March 2019, or maybe even April.

The 126-year average annual precipitation at Lake Arrowhead has been about 37.9” per Water Year. In the 22 El Niño years, annual precipitation has varied from a low of about 52” (137% of average) in 1965-66, to a high of 98” (258% of average) in 1968-69. A mild El Niño (54.42”) filled Lake Arrowhead for the first time in 1922. A much larger El Niño (79.45”) filled Lake Gregory in 1938.

In general, and on average, El Niño conditions bring about 12.4” of precipitation in December, 15.5” in January, 15.3” in February, and 12.4” in March. In December 2018, we got only 3.84”, and in January 2019 we got 12.09”, both below long-term El Niño averages. But, in February 2019, the 20.43” is above average.

Now, let’s look at precipitation in February 2019 and see what effect it has had on lake level. On Feb.1, lake level was at 5,099.89’, or 6.21’ (81.72”) below full at 5,106.7’. On March 1, lake level was at 5,105.69’, or only 1.01’ (12.12”) below full.

This rise of 5.8’ (69.6”) was due to 20.43” of precipitation falling directly on the lake plus surface runoff, or a rise of about 3.4” for every 1” of precipitation.

The 126-year average for March is about 6.54”. It will only take about 3½” of precipitation to fill the lake, so it will very likely occur this week of March 4-11.